Serotonin N-acetyltransferase (AANAT; EC 2.3.1.87) metabolizes serotonin into N-acetylserotonin (NAS). AANAT mRNA is expressed in the pineal gland and retina, and also in the rat brain. It was proposed that NAS could be a mediator of the antidepressant action of drugs, and it was shown that chronic but not acute treatment of rats with the antidepressant fluoxetine increases the content of AANAT mRNA in the rat hippocampus. Consequently, AANAT deficiency might be involved in the pathobiology of depression. C57BL/6J mice have a mutant AANAT gene and are considered AANAT-deficient, i.e., "knocked down" (compared with their normal counterparts, C3H/HeJ mice). In this study, we investigated whether AANAT mRNA is expressed in the brain of C57BL/6J and C3H/HeJ mice and whether those mice differ behaviorally, i.e., in a forced swimming test which is used to evaluate antidepressant drugs (such drugs shorten the time of immobility). We found that C3H/HeJ mice express in the brain normal AANAT mRNA, whereas C57BL/6J mice express mutated AANAT mRNA. The mutant, AANAT knocked down C57BL/6J mice displayed significantly longer times of immobility ("depression"). This difference was evident regardless of the circadian rhythm, i.e., both during the day and in the dark at night. Further studies are needed to fully characterize the behavioral significance of AANAT mutation and its possible link to depression.